This invention relates to a scanning electron microscope and more particularly, to a scanning electron microscope for detecting a secondary electron from above an objective lens.
To accomplish high resolution of a secondary electron image, aberration of a lens is preferably reduced as much as possible by reducing the focal length of an objective lens. Recently, therefore, a so-called "in-lens system" scanning electron microscope, which inserts a sample into the gap of an objective lens to reduce the focal length of the objective lens, has gained a wide application.
In the in-lens system scanning electron microscope, a secondary electron detector is disposed above the objective lens in order to detect a secondary electron from above the objective lens, and this secondary electron detector generates a secondary electron leading-out electric field.
In such an in-lens system scanning electron microscope, however, a primary electron beam emitted by a low acceleration voltage is bent by the secondary electron leading-out field and passes outside the optical axis of the objective lens, so that it receives an off-axis aberration of the lens and a secondary electron image having ,high resolution cannot be obtained.
The bend of the primary electron beam can be reduced by reducing the leading-out field of the secondary electron detector but in such a case, it becomes difficult to lead out a sufficient quantity of secondary electrons and a secondary electron image having high resolution cannot be obtained.
To eliminate such a problem, JP-A-59-134540 and JU-A-61-99960 propose the technique which corrects the bend of the primary electron beam by the secondary electron leading-out field by a deflection magnetic field generated by a correction excitation coil.
To reduce the influences of the secondary electron leading-out field on the primary electron beam, JP-A-61-131353 proposes a technique which covers the optical axis of an objective lens in the vicinity of a secondary electron detector by a shield electrode so as to shield the secondary electron leading-out field.
In accordance with the former system which corrects the bend of the primary electron beam by the deflection magnetic field, however, the correction excitation coil, too, must be adjusted whenever the acceleration voltage is adjusted, because a deflection magnetic field which is most suitable for the correction of the bend of the primary electron beam changes in accordance with the acceleration voltage. Hence, this proposal is not yet free from the problem that the operation is complicated and troublesome.
Another problem with this system is that since the primary electron beam is affected by two forces, that is, the deflection magnetic field and the leading-out electric field, the influences of these forces become irregular and accurate correction is difficult.
In accordance with the latter system which covers the optical axis of the objective lens by the shield electrode, on the other hand, observation by a low acceleration voltage is effective, it is true, but there occur the following problems during the observation by a high acceleration voltage.
When an acceleration voltage exceeds 20 kV, excitation of the objective lens becomes so strong that a low energy secondary electron of about a couple of electron.voltages generated from the sample is vigorously wound up by the magnetic field of the objective lens and the trajectory of the secondary electron is dispersed above the objective lens.
To efficiently collect the secondary electron generated from a wide range of the sample and to make observation over a wide visual field, therefore, the secondary electron leading-out field must be allowed to act sufficiently above the objective lens and the secondary electron must be collected in a broad space.
However, if the optical axis of the objective lens near the secondary electron detector is covered with the shield electrode, the intensity of the secondary electron leading-out field is reduced and the secondary electron can be collected only inside an extremely narrow and limited region on the sample, so that observation over a wide visual field cannot be conducted.